With normal vision, an individual is able to change focus for different distances. Ideally, an individual is able to focus on distant objects, referred to as distance vision, and on near objects, referred to as near vision. The optical system of the eye uses numerous muscles to focus for both distance and near vision. These muscles allow the eye to adjust focus when transitioning between distance vision and near vision. There are various responses involved in changing focus from distance vision to near vision. These include making the image clearer, the eyes turning in or out, and pupils changing size. If the eyes do not turn in enough with near vision, for example, then the individual would see double.
Presbyopia is a natural deterioration of near vision caused by loss of flexibility in the eye's lenses as one ages. This can be compensated by wearing “reading” glasses having lenses which correct refraction errors so that the eye does not have to focus as much. Often with presbyopia, the person does not need distance correction, but rather only near correction. To avoid continually taking the eyeglasses on and off, bifocals may be used. Bifocals enable the person to see at different distances. Particularly, bifocals are generally used when the eye can no longer correctly change focus from distance to near. Progressive addition lenses (PALs) include a distance portion and a smooth, graduating, continuous change in dioptic power to a near portion. PALs have no lines or edges visible between changes in dioptic power.
When a person transitions from looking through the distance portion to the near portion with bifocal lenses, the eyes do not need to focus to the same extent, because of the increase in dioptic power, as without the bifocals. As a result, there is a decrease in accommodative convergence, meaning that the eyes do not try to converge as much. This results in eye strain because the eyes subsequently use additional fusional convergence to converge for near.
The present invention is directed to solving one or more of the problems discussed above, in a novel and simple manner.